This invention relates to the addition and withdrawal of heterogeneous catalysts used in a catalytic process. In a particular embodiment, the invention relates to the addition and withdrawal of particulate catalyst used in the hydroprocessing of a heavy hydrocarbon residue.
By the incremental addition and withdrawal of catalyst to operating catalytic units, these units can be run on a continuous, rather than batch, basis. Downflow, fixed bed reactors are examples of batch flow, while upflow, ebullated bed reactors are preferably on a continuous basis. Thus, the addition and withdrawal of catalyst to such reactors is important.
U.S. Pat. Nos. 3,207,689 (Van Driesen), 3,336,217 (Meaux), 3,398,085 (Engle), 3,410,791 (Perry et al), 3,410,792 (Van Driesen and Stewart), 3,523,888 (Stewart and Van Driesen), and 3,547,809 (Ehrlich) teach prior methods of changing a catalyst charge. In the methods involving addition of catalyst to a reactor, it is customary to add the catalyst with gas or as a slurry of oil and catalyst to the reactor, apparently without regard to the temperature of the oil-catalyst mixture relative to the operating temperature of the reactor. If a large amount of mixture, at a temperature appreciably lower than that of the reactor, is added rapidly to the reactor, the reactor temperature will be reduced, thus upsetting some of the operating parameters. On the other hand, if a small stream of relatively cold catalyst-oil mixture is added without upsetting the reactor, it will take a long time to add an appreciable charge of new catalyst.
The typical prior art method of discharging used catalyst from a reactor comprises opening a discharge valve in the reactor and directing the effluent stream to a catalyst-oil separating vessel, with the driving force being the higher pressure in the reactor moving the catalyst-oil effluent to the separator. In this case, a change in pressure, in the reactor or in the discharge line, can result in a difference in flow rate to the separator, resulting in an approximation of the amount of catalyst discharged. Also, a great difference in pressure between the reactor and separator results in a rapid flow of the abrasive oil-catalyst mixture, which leads to valve and pipe erosion.